The invention is particularly concerned with providing a substantially constant current which can be used as a bias for a power supply detection circuit. Such power supply detection circuits are used in various applications for determining whether or not a power supply voltage has reached an adequate level for operation of the circuit. One particular application of a power supply detection circuit is for a flash memory chip. As the current consumption of a power supply detection circuit is a concern, since a chip's standby current is an important design parameter, it is a particular benefit to have a nearly constant current consumption for this application.
For flash memory chips which are capable of operating at 3.3 V or at 5 V, a power supply detection circuit must thus be capable of detecting a wide range of power supply voltages. Typically, the target operation range is between 2 V and 5.5 V. Moreover, these chips operate within a wide temperature range, for example of between minus 55.degree. C. to 125.degree. C. This means that it is very difficult to obtain a substantially constant bias current with conventional circuits.
A wide variation in the bias current can give problems because circuit parameters such as operation speed can then vary widely so that it is difficult to obtain adequate performance under all conditions.
One known current generator circuit is shown in FIG. 1, which illustrates a simple current mirror. As is well known, this circuit comprises a current setting element in the form of a diode connected p-channel transistor 2 which is connected between a power supply rail Vcc and a first current mirror transistor 4. The current mirror transistor is connected to a second power supply rail Vss and has its gate connected to the gate of a second current mirror transistor 6. This current mirror transistor 6 provides the reference current Iref which is matched to (or a fixed multiple of) the current I set by the current setting transistor 2.
The drawback with this circuit is that the current set by the current setting transistor 2 will vary widely with variations in the supply voltage Vcc and temperature. Thus, the reference current Iref provided by the circuit will also vary with these parameters.